Offset-reduced zero-gauss-magnet with polepiece for differential, twist-insensitive magnetic speed sensors

ABSTRACT

A magnetic sensor module includes an axially polarized back-bias magnet having a body that radially extends from a magnet center axis and a bore extending along the magnet center axis. The magnet generates a radial bias magnetic field about a magnetization axis in a sensor plane. The magnetic field has a magnetic flux density of substantially zero along a perimeter of a zero-field closed loop located in the sensor plane. The magnetic sensor module includes a shim polepiece mechanically coupled to an end of the axially polarized back-bias magnet. The shim polepiece includes a second bore centered on a polepiece center axis that is aligned with the magnet center axis. The second bore extends through the shim polepiece along the polepiece center axis.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.17/235,434, filed Apr. 20, 2021 (now U.S. Pat. No. 11,442,074), which isincorporated herein by reference in its entirety.

FIELD

The present disclosure relates generally to sensing a wheel speed, and,more particularly, to magnetic speed sensors.

BACKGROUND

To measure wheel speed (e.g., in an automotive application) typically aferromagnetic wheel is used in combination with a magnetic sensitivesensor and a magnet mounted to the sensor. The sensor generatesoutput-pulses. A control unit counts the pulses and is able to calculatewheel-speed and actual angle of the rotating wheel, as well asoptionally determine the rotation direction of the wheel.

In camshaft sensing applications, a Hall monocell configuration may beused that enables output switching at the tooth edge of a toothed wheel.A z-magnetized back-bias sensor in combination with the Bz-sensitivemonocell sensor generates a sinusoidal signal as the ferrous targetwheel rotates in front of the sensor. A back-bias magnet produces astatic magnetic bias field at the sensing elements, which is deflectedand modulated when the tooth wheel rotates. The maximum amplitude isachieved when a tooth passes the sensor, while the minimum signal isachieved when the sensor faces a notch of the toothed wheel. Thus, thesensor device switches on the tooth edge.

A benefit in using a Hall monocell sensor is that the sensor istwist-insensitive such that the sensor will work independent from amounting position regardless of its rotational orientation around itsz-axis. Thus, an air-gap between the sensor module and the wheel can beadjusted during mounting using a screw. That is, twisting the sensormodule like a screw will adjust the air gap and the rotationalorientation of the sensor can be disregarded. Accordingly, the assemblytolerances are relaxed during mounting of the sensor due to thetwist-insensitivity.

On the downside, Hall monocell sensors have a disadvantage in terms ofstray-field robustness. Stray-fields are magnetic fields that areintroduced by external means located in the proximal environment of thesensor. For example, components located within a vehicle (e.g., forhybrid cars due to current rails driving high electrical currents closeto the sensing device or due to inductive battery charging) or acurrents flowing through a railway of a train system that generatesmagnetic fields may cause stray-field disturbance.

Alternative to the Hall monocell sensor, differential Hall sensingelements may be used to increase the stray-field robustness. In adifferential Hall sensor, two Hall plates are spaced apart. The outputsignal is calculated by subtracting the Bz signal of the first Hallplate from the Bz signal of the second Hall plate, and a homogeneousstray-field in the z-direction will cancel out due to the differentialcalculation.

The differential Hall signal has its signal maximum at the rising edgeof a tooth of the wheel and its signal minimum at the falling edge of atooth of the wheel. Thus, in contrast to the Hall monocell sensor, theoutput of the differential Hall sensor switches on the tooth center andthe notch center.

However, because the switching point is different, a vehicle'selectronic control unit (ECU) needs to be reconfigured to adjust theswitching point. Furthermore, another disadvantage of the differentialHall sensor is that it is not twist-insensitive. Twisting the sensormodule around its z-axis, will result in a decreasing signal. The worstcase is a twist angle of 90°, where both Hall plates sense the sameBz-field. In this case no differential signal is available and thesensor is not able to detect a tooth or a notch.

In addition to Hall sensors, including lateral and vertical Hall sensingelements, there exists various magnetic sensing technologies exploitingdifferent kind of magnetic effects, included magneto-resistive sensingelements, often referred to as XMR sensors which is a collective termfor anisotropic magneto-resistive (AMR), giant magneto-resistive (GMR),tunneling magneto-resistive (TMR), etc. In all mentioned technologies,the electrical and or magnetic performance is limited either bysensitivity, linearity, or saturation effects. XMR technologies inparticular have a limited linear range of desired operation. Typically,the linear region ranges from a view single digit millitesla up to +−100millitesla. Therefore, the magnetic offset at the sensing elements fromthe back-bias magnet needs to be sufficiently low to avoid saturation ofthe sensing elements and ensure proper operation of the sensor. However,ensuring that the sensing elements are placed in a low magnetic offsetregion of a back-bias magnetic field is highly sensitive to productionand assembly tolerances.

Magnets are subject to production error. For example, a magnetizationdirection of an axially polarized back-bias magnet may be tilted awayfrom a center, axial axis instead of being aligned therewith. Themagnetization tilt changes the location of the low magnetic offsetregion, increasing the risk that the sensing elements will be placed ina back-bias magnetic field that is outside of its linear region. Themagnetization tilt becomes even more pronounced and problematic withstrong back-bias magnets. In addition, the placement of the sensorelements are subject to assembly error, where a slight deviation fromtheir ideal placement may result in the sensing elements being placed ina back-bias magnetic field that is outside of its linear region. Usingstronger back-bias magnets may be desirable as they allow for largerairgaps between the sensor and the target wheel. They are also morerobust against stray-fields.

Accordingly, due to the assembly and magnetization tilt errors, theoverall magnetic offset might become too high and drive the sensingelements to saturation. In that case, the overall sensor performance isreduced. In worst case, the sensor modules need to be discarded and themanufacturing yield is reduced.

Therefore, an improved device that is stray-field robust,twist-insensitive (i.e., twist independent), and has higher assembly andproduction tolerances may be desirable.

SUMMARY

Magnetic sensor modules, systems and methods are provided, configured todetect a rotation of an object, and, and more particularly, to detect aspeed of rotation of an object.

One of more embodiments provide a magnetic sensor module configured todetect a rotation of an object. The magnetic sensor module includes anaxially polarized back-bias magnet comprising a magnet body that extendsin a radial direction from an inner sidewall to an outer sidewall and afirst bore that defines the inner sidewall, wherein the axiallypolarized back-bias magnet generates a radial bias magnetic in-planefield about a magnetization axis of the axially polarized back-biasmagnet in a sensor plane that is orthogonal to an extension of a magnetcenter axis of the axially polarized back-bias, wherein the first boreis centered on the magnet center axis and extends along the magnetcenter axis in an axial direction of the axially polarized back-biasmagnet, wherein the radial bias magnetic in-plane field has a magneticflux density of substantially zero along the extension of themagnetization axis and at a perimeter of a zero-field closed looplocated in the sensor plane, wherein the perimeter of the zero-fieldclosed loop encircles the extension of the magnet center axis, whereinthe axially polarized back-bias magnet comprises a first end and asecond end arranged opposite to the first end in the axial direction;and a shim polepiece mechanically coupled to the first end of theaxially polarized back-bias magnet, wherein the shim polepiece comprisesa second bore centered on a polepiece center axis that is aligned withthe magnet center axis, wherein the second bore extends through the shimpolepiece along the polepiece center axis and is congruent with thefirst bore.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are described herein making reference to the appendeddrawings.

FIGS. 1A and 1B illustrate a magnetic field sensing principle of atoothed wheel according to one or more embodiments;

FIG. 2 shows a plan view (top view) of an axially polarized back-biasmagnet and its in-plane magnetic field distribution according to one ormore embodiments.

FIG. 3 illustrates a graph that plots the in-plane magnetic field Bxy toa radial distance from center axis of a back-bias magnet at differentminimum distances taken in a z-direction according to one or moreembodiments.

FIG. 4 illustrates a graph that plots a dependency of in-plane magneticfield Bxy on bore diameter according to one or more embodiments.

FIG. 5 illustrates a plan view of a sensor module, including a sensorarrangement arranged over a back-bias magnet, according to one or moreembodiments.

FIG. 6 illustrates a back-bias magnet that is an axially polarizedcylinder and includes a bore according to one or more embodiments.

FIG. 7 illustrates a speed sensing system, including a sensor module,according to one or more embodiments.

FIG. 8 illustrates a plan view of another sensor module, including asensor arrangement arranged over a back-bias magnet, according to one ormore embodiments.

FIG. 9 illustrates a plan view of another sensor module, including asensor arrangement arranged over a back-bias magnet, according to one ormore embodiments.

FIGS. 10A-10D illustrate sensor signals of a sensor elements of a sensorshown in FIG. 5 verse a rotation angle of a target wheel according toone or more embodiments.

FIG. 10E illustrates output sensor signals of a sensor circuit of asensor shown in FIG. 5 verse a rotation angle of a target wheelaccording to one or more embodiments.

FIG. 10F illustrates output sensor signals for different TwistInsensitive Mounting (TIM) angles of the sensor module shown in FIG. 5verse a rotation angle of a target wheel according to one or moreembodiments.

FIG. 11 illustrates a flow diagram of a method of measuring a rotationalspeed of a rotating member by a magnetic sensor according to one or moreembodiments.

FIG. 12 illustrates a sensor module 6 including a polepiece according toone or more embodiments.

FIG. 13A illustrates the in-plane magnetic field components Bx and By ofan ideal axially polarized back-bias magnet comprising a bore andwithout the polepiece.

FIG. 13B illustrates the in-plane magnetic field components Bx and By ofa tilted axially polarized back-bias magnet comprising a bore andwithout the polepiece.

FIG. 14A illustrates the in-plane magnetic field components Bx and By ofan ideal axially polarized back-bias magnet comprising a bore and withthe polepiece.

FIG. 14B illustrates the in-plane magnetic field components Bx and By ofa tilted axially polarized back-bias magnet comprising a bore and withthe polepiece.

FIG. 15A illustrates the in-plane magnetic field components Bx and By ofan ideal axially polarized back-bias magnet comprising a bore and with aconventional solid polepiece.

FIG. 15B illustrates the in-plane magnetic field components Bx and By ofa tilted axially polarized back-bias magnet comprising a bore and with aconventional solid polepiece.

DETAILED DESCRIPTION

In the following, details are set forth to provide a more thoroughexplanation of the exemplary embodiments. However, it will be apparentto those skilled in the art that embodiments may be practiced withoutthese specific details. In other instances, well-known structures anddevices are shown in block diagram form or in a schematic view ratherthan in detail in order to avoid obscuring the embodiments. In addition,features of the different embodiments described hereinafter may becombined with each other, unless specifically noted otherwise. It isalso to be understood that other embodiments may be utilized andstructural or logical changes may be made without departing from thescope defined by the claims. The following detailed description,therefore, is not to be taken in a limiting sense.

Further, equivalent or like elements or elements with equivalent or likefunctionality are denoted in the following description with equivalentor like reference numerals. As the same or functionally equivalentelements are given the same reference numbers in the figures, a repeateddescription for elements provided with the same reference numbers may beomitted. Hence, descriptions provided for elements having the same orlike reference numbers are mutually exchangeable.

Directional terminology, such as “top”, “bottom”, “above”, “below”,“front”, “back”, “behind”, “leading”, “trailing”, “over”, “under”, etc.,may be used with reference to the orientation of the figures and/orelements being described. Because the embodiments can be positioned in anumber of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. In someinstances, directional terminology may be exchanged with equivalentdirectional terminology based on the orientation of an embodiment solong as the general directional relationships between elements, and thegeneral purpose thereof, is maintained.

In the present disclosure, expressions including ordinal numbers, suchas “first”, “second”, and/or the like, may modify various elements.However, such elements are not limited by the above expressions. Forexample, the above expressions do not limit the sequence and/orimportance of the elements. The above expressions are used merely forthe purpose of distinguishing an element from the other elements. Forexample, a first box and a second box indicate different boxes, althoughboth are boxes. For further example, a first element could be termed asecond element, and similarly, a second element could also be termed afirst element without departing from the scope of the presentdisclosure.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

In embodiments described herein or shown in the drawings, any directelectrical connection or coupling, i.e., any connection or couplingwithout additional intervening elements, may also be implemented by anindirect connection or coupling, i.e., a connection or coupling with oneor more additional intervening elements, or vice versa, as long as thegeneral purpose of the connection or coupling, for example, to transmita certain kind of signal or to transmit a certain kind of information,is essentially maintained. Features from different embodiments may becombined to form further embodiments. For example, variations ormodifications described with respect to one of the embodiments may alsobe applicable to other embodiments unless noted to the contrary.

Depending on certain implementation requirements, a storage medium mayinclude a RAM, a ROM, a PROM, an EPROM, an EEPROM, a FLASH memory, orany other medium having electronically readable control signals storedthereon, which cooperate (or are capable of cooperating) with aprogrammable computer system such that the respective method isperformed. Therefore, a storage medium may be regarded as anon-transitory storage medium that is computer readable.

Additionally, instructions may be executed by one or more processors,such as one or more central processing units (CPU), digital signalprocessors (DSPs), general purpose microprocessors, application specificintegrated circuits (ASICs), field programmable logic arrays (FPGAs), orother equivalent integrated or discrete logic circuitry. Accordingly,the term “processor,” as used herein refers to any of the foregoingstructure or any other structure suitable for implementation of thetechniques described herein. In addition, in some aspects, thefunctionality described herein may be provided within dedicated hardwareand/or software modules. Also, the techniques could be fully implementedin one or more circuits or logic elements. A “controller,” including oneor more processors, may use electrical signals and digital algorithms toperform its receptive, analytic, and control functions, which mayfurther include corrective functions.

Signal conditioning, as used herein, refers to manipulating an analogsignal in such a way that the signal meets the requirements of a nextstage for further processing. Signal conditioning may include convertingfrom analog to digital (e.g., via an analog-to-digital converter),amplification, filtering, converting, biasing, range matching, isolationand any other processes required to make a sensor output suitable forprocessing after conditioning.

Embodiments relate to sensors and sensor systems, and to obtaininginformation about sensors and sensor systems. A sensor may refer to acomponent which converts a physical quantity to be measured to anelectric signal, for example, a current signal or a voltage signal. Thephysical quantity may for example comprise a magnetic field, an electricfield, a pressure, a force, a current or a voltage, but is not limitedthereto. A sensor device, as described herein, may be a speed sensorthat measures a rotational speed of an object, such as a toothed wheel.

A magnetic field sensor, for example, includes one or more magneticfield sensor elements that measure one or more characteristics of amagnetic field (e.g., an amount of magnetic field flux density, a fieldstrength, a field angle, a field direction, a field orientation, etc.).The magnetic field may be produced by a magnet, a current-carryingconductor (e.g., a wire), the Earth, or other magnetic field source.Each magnetic field sensor element is configured to generate a sensorsignal (e.g., a voltage signal) in response to one or more magneticfields impinging on the sensor element. Thus, a sensor signal isindicative of the magnitude and/or the orientation of the magnetic fieldimpinging on the sensor element.

According to one or more embodiments, a magnetic field sensor and asensor circuit are both accommodated (i.e., integrated) in the same chippackage (e.g., a plastic encapsulated package, such as leaded package orleadless package, or a surface mounted device (SMD)-package). This chippackage is also referred to as sensor package. The sensor package may becombined with a back-bias magnet to form a sensor module, sensor device,or the like.

One or more magnetic field sensor elements, or for short a magneticfield sensors, included in the sensor package is thus exposed to themagnetic field, and the sensor signal (e.g., a voltage signal) providedby each magnetic field sensor element is proportional to the magnitudeof the magnetic field, for example. Further, it will be appreciated thatthe terms “sensor” and “sensing element” may be used interchangeablythroughout this description, and the terms “sensor signal” and“measurement value” may be used interchangeably throughout thisdescription.

The sensor circuit may be referred to as a signal processing circuitand/or a signal conditioning circuit that receives the signal (i.e.,sensor signal) from the magnetic field sensor element in the form of rawmeasurement data and derives, from the sensor signal, a measurementsignal that represents the magnetic field. The sensor circuit mayinclude a digital converter (ADC) that converts the analog signal fromthe one or more sensor elements to a digital signal. The sensor circuitmay also include a digital signal processor (DSP) that performs someprocessing on the digital signal, to be discussed below. Therefore, thesensor package comprises a circuit which conditions and amplifies thesmall signal of the magnetic field sensor via signal processing and/orconditioning.

A sensor device, as used herein, may refer to a device which includes asensor and sensor circuit as described above. A sensor device may beintegrated on a single semiconductor die (e.g., silicon die or chip),although, in other embodiments, a plurality of dies may be used forimplementing a sensor device. Thus, the sensor and the sensor circuitare disposed on either the same semiconductor die or on multiple dies inthe same package. For example, the sensor might be on one die and thesensor circuit on another die such that they are electrically connectedto each other within the package. In this case, the dies may becomprised of the same or different semiconductor materials, such as GaAsand Si, or the sensor might be sputtered to a ceramic or glass platelet,which is not a semiconductor.

Magnetic field sensor elements include, but is not limited to, verticalHall effect devices, or magneto-resistive sensors, often referred to asXMR sensors which is a collective term for anisotropic magneto-resistive(AMR), giant magneto-resistive (GMR), tunneling magneto-resistive (TMR),etc.

FIGS. 1A and 1B illustrate a magnetic field sensing principle of atoothed wheel 1 that has alternating teeth 2 and notches 3 according toone or more embodiments. In particular, the toothed wheel 1 may be madeof a ferromagnetic material (e.g., iron) that attracts magnetic fields.In addition, a sensor arrangement 4 is configured to sense a magneticfield produced by an axially polarized (axially magnetized) back-biasmagnet 5, where the sensor arrangement 4 and the back-bias magnet 5comprise a sensor module 6. The back-bias magnet 5 may be mounted to theback or rear side of the sensor die (i.e., the sensor arrangement 4)relative to the toothed wheel 1, which is placed in front of the sensordie. The sensor arrangement 4 may generally be referred to herein as asensor and may be disposed in a sensor package. The axially polarizedmagnet 5 creates a radially symmetric bias magnetic field in the sensorpackage plane (i.e., chip plane).

A cylinder with an axial bore or an axial cavity may be used as theaxially polarized magnet 5. The magnetic in-plane field (i.e.sqrt(Bx²+By²)) in the sensor plane (i.e., chip plane) produced by theaxially polarized magnet 5 is zero at the center of the magnet (i.e., atits center axis), increases in a radial direction from the center axis(e.g., the z-axis as shown), then, due to the axial bore or the axialcavity, the magnetic in-plane field decreases back to zero at azero-field offset point, and then increases again in the radialdirection from the zero-field offset point. Thus, an in-plane magneticfield is created by the magnet 5 with a zero magnetic flux density atthe center axis and a zero magnetic flux density located at the zerofield offset point. The zero-field offset point located at a radial(i.e., lateral) distance from the center axis and circumferentiallysurrounds the center axis. In other words, the zero-field offset pointis located on a circumference of a zero-field circle 12 that isconcentric to the center axis (i.e., the z-axis) and located on thesensor plane of the sensor elements. The in-plane magnetic flux densityof the magnetic field produced by the magnet 5 is zero along thecircumference of this zero-field circle 12 and along the center axis ofthe cylinder. There will always be a zero transition of the radial fieldcomponent due to the bore and this transition occurs at the zero-fieldcircle 12. The zero-field circle 12 may also be referred to as azero-field closed loop that encircles the center axis 9.

The radial distance at which the circumference of the zero-field circleis located depends at least on the diameter of the bore or cavity and aminimum distance between the sensor arrangement 4 (i.e., the sensorplane) and the back-bias magnet 5.

Here, the sensor plane(s) of the sensor elements within the sensorarrangement 4 are arranged parallel to the in-plane components of themagnetic field. The sensor planes, as shown in FIGS. 1A and 1B, arealigned in the x and y-directions, perpendicular to each other, andrepresent the sensitivity-axis of the sensor elements such that thesensor elements are sensitive to the in-plane magnetic field componentBx (i.e., the magnetic field in the x-plane) or to the in-plane magneticfield component By (i.e., the magnetic field in the y-plane) of thesensor arrangement 4. Thus, the sensor elements are sensitive to theradially symmetric bias magnetic field produced by the magnet 5.

FIG. 1A shows a tooth 2 of wheel 1 passing the sensor module 6. In thisinstance, the magnetic field lines of the radially symmetric biasmagnetic field produced by the back-bias magnet 5 are pulled in thez-direction towards the tooth 2. Thus, the magnetic field lines arepulled away from the x and y-axes (i.e., the sensor planes) and thesensed magnetic field strength in the x and y-directions is reduced suchthat a minimum field strength is detected at the center of the tooth 2.This may differ in real-world applications where the minimum may notoccur exactly at the center due to assembly tolerances, but the minimumfield strength should be detected substantially at the center of thetooth 2.

Conversely, FIG. 1B shows a notch 3 of wheel 1 passing the sensor module6. In this instance, the magnetic field lines of the radially symmetricbias magnetic field produced by the back-bias magnet 5 are not pulled(or less pulled) in the z-direction towards the notch 3. Thus, themagnetic field lines remain concentrated relative to the x and y-axes(i.e., the sensor planes) and the sensed magnetic field strength in thex and y-directions are at a maximum at the center of the notch 3. Thismay differ in real-world applications where the maximum may not occurexactly at the center, but the maximum field strength should be detectedsubstantially at the center of the notch 3.

As the wheel 1 rotates, the teeth 2 and notches 3 alternate past thesensor module 6 and the sensor elements within the sensor arrangement 4sense a change in the x-axis and y-axis magnetic field strength thatvaries as a sinusoidal waveform (i.e., as a signal modulation), thefrequency of which corresponds to a speed of rotation of the wheel,which further corresponds to a speed of rotation of a drive shaft (e.g.,camshaft) that drives the rotation of the wheel. Thus, the sensorcircuit of the sensor arrangement 4 that receives signals (i.e., sensorsignals) from the magnetic field sensor elements and derives, from thesensor signals, a measurement signal that represents the magnetic fieldas a signal modulation. The measurement signal may then be output as anoutput signal to an external controller, control unit or processor(e.g., an ECU). The external device counts the pulses of the outputsignal and is able to calculate wheel-speed and an actual angle of therotating wheel.

Further embodiments relate to a twist-insensitive, differential sensorconcept in combination with a zero-gauss back-bias magnet 5.Specifically, back-bias magnet 5 comprises a center axis with a bore 10or a cavity that extends axially along the center axis. The bore mayextend in the axial direction fully from one end face of the back-biasmagnet 5 to the opposite end face of the back-bias magnet 5.Alternatively, the bore 10 may be open at one end face and closed at theopposite end face of the back-bias magnet 5. In this case, the bore 10may extend from one end face (i.e., the open end face) of the back-biasmagnet 5 partially towards the opposite, closed end face of theback-bias magnet 5. To be discussed below, a shim polepiece 30 is placedon the open end face for this type of magnet, which is also the senorside of the back-bias magnet 5.

In either case, the back-bias magnet 5 is an axially polarized back-biasmagnet that generates a radially symmetric bias magnetic field about acenter axis of the axially polarized back-bias magnet in a sensor plane(e.g., in an x-y plane). The back-bias magnet 5 may be an axiallypolarized ring magnet (i.e., a cylinder magnet with an axial boreextending therethrough), an axially polarized conical magnet with anaxial bore, an axially polarized cubic magnet with an axial bore, andthe like.

The sensor die (i.e., the sensor arrangement 4) comprises a minimum oftwo sensing elements with at least two of the sensing elements locatedat zero-field offset points that are located on a circumference of azero-field circle that is concentric to the center axis. In one case,all sensing elements (i.e., two or more) may be located at zero-fieldoffset points that are located on the circumference of the zero-fieldcircle. In another case, one of the sensor elements may be located onthe center axis while the other sensor elements may be located atzero-field offset points that are located on the circumference of thezero-field circle. For those sensing elements located on thecircumference of the zero-field circle, they are arranged at equidistantangles on the circle with radius r to provide radial symmetry.

The origin of the zero-field circle is aligned with the center axis ofthe back-bias magnet 5 (i.e., the center of the bore). In this way allsensing elements are exposed to the same low magnetic offset. Ideally,the magnetic offset is 0 mT. Therefore, the back-bias magnet 5 behaveslike a zero-gauss magnet. It produces essentially 0 mT magnetic field inthe sensor plane at the position of all sensing elements and thesensitive field directions.

The low or zero magnetic field offset offers some crucial advantages.First, in case of a sensor with saturation effects (limited linearrange), such as an)(MIR sensing element, a strong rare-earth magnet canbe used without saturating the sensing elements. In particular, XMRsensing elements have a limited linear range. Outside of this linearrange, the sensing element enters saturation and the sensor signalbecomes unreliable (e.g., it is no longer proportional to the sensedmagnetic field). Thus, saturation should be avoided. When)(MIR sensingelements are placed away from the zero-filed circle, the probability ofsaturation increases when a strong rare-earth magnet is used as theback-bias magnet 5.

The low or zero magnetic field offset enables higher stray-fieldrobustness as the signal-to-stray field ratio is increased. Second, thelow or zero magnetic field offset allows implementation of reliable TruePower On (TPO) feature. Furthermore, life-time drifts (aging effects ofa magnet) and a mismatch of magnet temperature coefficient and sensortemperature coefficient is less critical in case of low or zero magneticfield offsets. Further still, the low or zero magnetic field offsetenables the use of strong magnetic material (e.g., a rare-earth materialsuch as NdFeB or SmCo) without the risk of saturating the sensorelements. Using a strong magnetic material enables a high airgapcapability between the sensor and the wheel and allows for bettermanufacturing tolerances.

Additionally, additional advantages of the overall system can berealized, including enhanced stray-field robustness due to differentialsensing principle, Twist Insensitive Mounting (TIM) where thedifferential signal produced is independent from twisting around thez-axis, it is directly possible to sense the shape of the soft magneticpattern (i.e., tooth high, notch low), and it enables small diametermodules (e.g., the back-bias magnet diameter may be 5 mm or less).

FIG. 2 shows a plan view (top view) of an axially polarized back-biasmagnet and its in-plane magnetic field distribution according to one ormore embodiments. The back-bias magnet 5 includes an inner diameter 7defined by a bore 10 and further includes an outer diameter 8. Theback-bias magnet 5 includes a center axis 9 that extends orthogonal tothe x-y sensor plane and to which the inner diameter 7 and the outerdiameter 8 are concentric. The back-bias magnet 5 generates a radiallysymmetric bias magnetic field Bxy about the center axis 9 in the x-ysensor plane, also referred to as an in-plane magnetic field Bxy. Thez-axis extends out of the sensor plane, and is consequently referred toas out-of-plane.

As can be seen from the in-plane magnetic field distribution, themagnetic flux density of the magnetic field Bxy is zero at the center ofthe bore 10 at the center axis 9. The magnetic flux density of themagnetic field Bxy is also zero at a zero-field offset point 11 locatedon a circumference of a zero-field circle 12 that is concentric to thecenter axis 9. Because the magnetic field Bxy is radially symmetricabout the center axis 9, each point at an equal radial distance from thecenter axis 9 experience the same magnetic flux density. Thus, eachpoint on the zero-field circle 12 experiences a zero magnetic fieldoffset (i.e., zero teslas). Sensor elements are to be place at orsubstantially at (to allow for manufacturing and assembly tolerances)the zero-field circlet 12 such that they are exposed to an in-planemagnetic flux density (i.e., magnetic field offset) of zero or asubstantially zero (i.e., <15 mT). In other words, due to considerationsof manufacturing and assembly tolerances, one or more sensor elementsmay be placed proximate to the zero-field circlet 12 where some (i.e.,less than 15 mT) residual offset field exists.

The zero-field circle 12, and, thus, the zero-field offset point 11, islocated at a radial distance between the inner diameter 7 and the outerdiameter 8 of the back-bias magnet 5. In other words, the zero-fieldcircle 12 vertically overlaps with the magnet body of the back-biasmagnet 5 when viewed from a top or plan view, where magnet body extendsradially from the inner diameter 7 (i.e., an inner sidewall) to theouter diameter 8 (i.e., an outer sidewall).

The actual location of the zero-field offset point 11 depends on thediameter of the bore 10 and a minimum distance between the sensorarrangement 4 (i.e., the sensor plane) and the back-bias magnet 5. Thesensing elements of the sensor may be placed at a location with lowmagnetic offset defined as being less than 15 mT, but ideally are placedat a location where the magnetic field offset is zero (i.e., at thecenter axis 9 and at the zero-field circle 12).

FIG. 3 illustrates a graph that plots the in-plane magnetic field Bxy toa radial distance from center axis of a back-bias magnet at differentminimum distances taken in a z-direction according to one or moreembodiments. There will always be a zero transition of the radial fieldcomponent due to the bore 10 and this transition occurs at thezero-field circle 12. In particular, a first curve pertains to thein-plane magnetic field Bxy at a first minimum distance of 0.7 mm fromthe back-bias magnet 5 and a second curve pertains to the in-planemagnetic field Bxy at a second minimum distance of 0.3 mm from theback-bias magnet 5. Each minimum distance is a z-distance from a nearestend of the back-bias magnet 5 relative to the sensor arrangement 4 thatdefines the sensor plane. The diameter of the bore 10 is the same forboth curves.

It is noted that the bore 10 axially extends from one end face of theback-bias magnet 5 to an opposite, second end face of the back-biasmagnet 5 with the ends of the bore 10 exposed. However, in other cases,the bore 10 may be completely enclosed within the magnet body of themagnet 5 as long as a zero magnetic field offset (i.e., zero in-planemagnetic flux density) is created at both the center axis 9 and thezero-field circle 12.

Turning back to FIG. 3 , the first curve shows a zero magnetic fieldoffset (i.e., magnetic flux density) is present at the center axis 9(i.e., 0 mm) and about 1.13 mm, where the inner diameter is located at aradial distance of 1 mm from the center axis 9 and the outer diameter islocated at a radial distance of 2.5 mm from the center axis 9. Incontrast, the second curve shows a zero magnetic field offset is presentat the center axis 9 (i.e., 0 mm) and about 1.38 mm. Thus, minimumdistance at which the sensor arrangement 4 is arranged can be configuredaccording to the desired sensor pitch between sensing elements. It isalso noted that as a result of the increased minimum distance, the firstcurve has a gradient of the field vs. radial position that is much lowercompared to the second curve.

FIG. 4 illustrates a graph that plots a dependency of in-plane magneticfield Bxy on bore diameter according to one or more embodiments. Inparticular, several curves are provided for the in-plane magnetic fieldBxy where the diameter of bore 10 varies from 1.6 mm to 2.8 mm byincrements of 0.2 mm. As can be seen, a zero magnetic field offset ispresent at the center axis 9 (i.e., 0 mm) for all curves. However, theradial distance of the zero-field circle 12 where the second zeromagnetic field offset is present increases with increasing borediameter.

For a constant magnet outer dimension and constant minimum distance (inthis case 0.7 mm z-distance of the sensor arrangement to the magnet 5),the position of the zero-field offset can be adjusted by changing thebore diameter. A smaller diameter leads to smaller sensor pitch. Oftendue to module assembly constrains, the outer magnet diameter is limited.Thus, for a fixed outer diameter, the position of the zero-field offsetcan be adjusted with the bore diameter. Note that this sensor alsorealizes small diameter modules, as the magnet 5 has an outer diameterof only 5 mm due to the placement of sensing elements on orsubstantially on the zero-field circle 12. The reduced magnet size in animprovement over other sensor modules that do not have such aconfiguration. However, it will be appreciated that the embodimentsdescribed herein are not limited by any specific dimension of the magnet5.

FIG. 5 illustrates a plan view of a sensor module, including a sensorarrangement arranged over a back-bias magnet, according to one or moreembodiments. As used herein, the sensor arrangement 4 may also bereferred to as a sensor chip layout, single die sensor or magneticsensor and includes at least two magnetic field sensor elements. In thiscase, four magnetic field sensor elements 20L, 20R, 20U, and 20D(collectively referred to as sensor elements 20) are provided. Thesensor chip also includes a sensor circuit 21. The sensor elements 20are arranged on a circumference of the zero-field circle 12 withequidistant spacing from each other. Thus, the sensor elements 20 arespatially distributed equally about a center axis 9 of the zero-fieldcircle 12 such that all sensor elements 20 are exposed to substantiallythe same (due to typical assembly tolerances of 3%), or exactly the samemagnetic field working point. The zero-field circle 12 has a diameter D.

The magnetic flux density of the magnetic field changes symmetrically ina radial direction from the center axis (e.g., from the z-axis as shown)as described above. For example, the magnetic flux density of theradially symmetric magnetic in-plane field produced by the axiallypolarized magnet 5 is zero at the center of the magnet (i.e., at itscenter axis) and, due to the bore 10, also at the zero-field circle 12.Aside from those two locations, the radially symmetric magnetic fluxdensity is non-zero in accordance with aforementioned details. Thus, thecenter point of the zero-field circle 12 coincides with the center axis9 of the magnet 5 so that each sensor element 20 is exposed tosubstantially the same (due to typical assembly tolerances of 3%), orexactly the same magnetic in-plane field (i.e., magnetic offset).

Each point on the zero-field circle 12 experiences a zero magnetic fluxdensity. In a preferred case, all sensor elements 20 are exposed to azero in-plane magnetic field Bxy. As a result of their arrangement onthe zero-field circle 12 that is generated due to the bore 10 orsubstantially proximate to the zero-field circle 12, all sensor elementsare always operated in linear mode (i.e., non-saturated mode) regardlessof the overall magnetization strength of the magnet 5. It is noted thatthe out-of-plane magnetic field Bz is not a consideration since thesensor elements 20 are configured via their respective sensing axes tomeasure an in-plane magnetic field component (i.e., either the Bxmagnetic field component or the By magnetic field component).

The sensor elements 20 may be, for example, single-axis or multi-axisXMR sensor elements that have a sensing axis utilized for the speedsensor that is aligned with one of the in-plane magnetic fieldcomponents Bx or By. Here, as similarly described above with referenceto FIGS. 1A and 1B, it is assumed for this example that the back-biasmagnet 5 produces a radially symmetric bias magnetic field.Additionally, each sensor's transfer function has a high linear range(minimum of +/−25 mT or greater) and is in a wide range independent frombias fields. That is, each sensor element 20 is sensitive to a firstmagnetic in-plane field component (e.g., a Bx component) and, at thesame time, it is independent from (or insensitive to) a second,different magnetic in-plan field component (e.g., a By component).

The arrows on each sensor element 20 indicate a direction of thereference layer of the sensor element 20 having a reference directionsuch that the reference direction of sensor elements 20L, 20R are thesame and the reference direction of sensor elements 20U, 20D are thesame. Thus, sensor elements 20L and 20R share their same referencedirection, and sensor elements 20U and 20D share their own samereference direction. Moreover, the sign of the pairwise referencedirections is also invertible. This means in another embodiment, sensorelements 20L and 20R may also be sensitive to the −Bx direction, whilesensor elements 20U and 20D may be sensitive to the −By direction.Accordingly, if the magnetic field points exactly in the same directionas the reference direction, the resistance of the XMR sensor element isat a maximum, and, if the magnetic field points exactly in the oppositedirection as the reference direction, the resistance of the XMR sensorelement is at a minimum.

According to this example, oppositely disposed sensor elements 20L and20R may have a sensing axis in the x-direction configured for sensingthe in-plane magnetic field component Bx (i.e., sensitive to magneticfields in the x-plane). Similarly, oppositely disposed sensor elements20U and 20D may have a sensing axis in the y-direction configured forsensing the in-plane magnetic field component By (i.e., sensitive tomagnetic fields in the y-plane).

From the four sensors 20, two differential signals are obtained.Depending on the type or use of the sensor the change of magnetic fieldis translated into a change of resistance, current, or voltage. A firstdifferential signal (e.g., a speed signal) may be calculated asΔ1=Left−Right or in a magnetic field representation, Δ1=BxLeft−BxRight.A second differential signal (e.g., a direction signal), phase shiftedby 90° from the first differential signal may be calculated asΔ2=Down−Up or in a magnetic field representation, Δ2=ByDown−ByUp.

The sensor signals of each sensor element 20 is provided to the sensorcircuit 21 that calculates the two differential signals Δ1 and Δ2 and/oran output signal using a differential calculation that cancels out thehomogeneous stray-fields in the x and y-directions, and out-of-planemagnetic field components do not affect the output signal (i.e., thesensor output). The output signal R_(OUT) or V_(OUT) is calculated, forexample, by the following equations:R _(OUT) =R _(LEFT) −R _(RIGHT)−(R _(UP) −R _(DOWN))  (1), orR _(OUT) =R _(LEFT) −R _(RIGHT)(R _(DOWN) −R _(UP))  (2),V _(OUT) =V _(LEFT) −V _(RIGHT)−(V _(UP) −V _(DOWN))  (3), orV _(OUT) =V _(LEFT) −V _(RIGHT)(V _(DOWN) −V _(UP))  (4).Here, R_(LEFT) corresponds to a resistance value of sensor element 20L,R_(RIGHT) corresponds to a resistance value of sensor element 20R,R_(UP) corresponds to a resistance value of sensor element 20U, andR_(DOWN) corresponds to a resistance value of sensor element 20D.Furthermore, V_(LEFT) corresponds to a voltage value of sensor element20L, V_(RIGHT) corresponds to a voltage value of sensor element 20R,V_(UP) corresponds to a voltage value of sensor element 20U, andV_(DOWN) corresponds to a voltage value of sensor element 20D. Equations(1), (2), (3), and (4) can be generalized as follows:SE_(OUT)=(SE_(A)−SE_(B))+(SE_(C)−SE_(D))  (5), orSE_(OUT)=Δ1+Δ2  (6),where SE corresponds to sensor element, and SE_(A) and SE_(B) correspondto a first pair of oppositely disposed sensor elements, and SE_(C) andSE_(D) correspond to a second pair of oppositely disposed sensorelements.

As the sensor elements 20 are)(MIR sensor elements, the resistancevalues change depending on the magnetic field strength in the directionof the sensing axis, and the resistance values of the XMR sensorelements may be detected by the sensor circuit 21 or may be output fromthe senor element as a voltage value that is representative of theresistance value (i.e., the voltage value changes as the resistancevalue changes). In the former case, the resistance value is output as asensor signal, and, in the latter case, the voltage value is output as asensor signal, however, the sensor signal is not limited thereto. Thus,external stray-fields in the sensor plane will cancel out due to thedifferential calculus and out-of-plane magnetic field components do notaffect the sensor output. When mounted in front of a ferromagnetictarget wheel 1, the sensor output is independent from the sensor'stwisting around the magnet axis 9. As a result, the output signalSE_(OUT) will not change its phase, which means it is twist-independent.

Alternatively, the sensor elements 10 may be, for example, vertical Hallsensor elements that have a sensing axis utilized for the speed sensorthat is aligned with one of the in-plane magnetic field components Bx orBy. In vertical Hall sensor elements, voltage values output by thesensor elements 10 change according to the magnetic field strength inthe direction of the sensing axis. Thus, external stray-fields in thesensor plane will cancel out due to the differential calculus andout-of-plane magnetic field components do not affect the sensor output.

Thus, oppositely disposed sensor elements 20L and 20R may have a sensingaxis aligned in the x-direction configured for sensing the in-planemagnetic field component Bx (i.e., sensitive to magnetic fields in thex-plane). Similarly, oppositely disposed sensor elements 20U and 20D mayhave a sensing axis aligned in the y-direction configured for sensingthe in-plane magnetic field component By (i.e., sensitive to magneticfields in the y-plane).

In addition, the sensor module 6 includes an axially polarized cylindermagnet 5, where its center axis 9 points towards the wheel 1 andcoincides with the center of the inner diameter 7, the center of thezero-field circle 12, and the center or the outer diameter 8. Thus, themagnet creates a radially symmetric bias magnetic field in the sensorplane such that each sensor element 20 is exposed to substantially thesame (due to typical assembly tolerances of 3%), or exactly the samemagnetic field working point. The magnet may be any shape that producesa radially symmetric magnetic field (e.g., cylinder, cube, etc.) andfurther includes a bore (open-ended or closed-ended) that produces thezero-field circle 12.

For example, FIG. 6 illustrates a back-bias magnet 5 that is an axiallypolarized cylinder and includes a bore 10 according to one or moreembodiments. FIG. 6 further shows the in-plane magnetic fielddistribution in the sensor plane. The magnetic flux density is zero inthe center of the plane and changes in the radial direction in thesensor plane such that it is also zero at the zero-field circle 12.Thus, due to the radially symmetric field distribution, all four sensorelements 20 are exposed to substantially the same (due to typicalassembly tolerances of 3%), or exactly the same magnetic field workingpoint.

FIG. 7 illustrates a speed sensing system 400, including a sensor module6, according to one or more embodiments. In particular, a portion ofwheel 1 is shown with an air gap between the wheel 1 and the sensormodule 6, and, more particularly, between the wheel 1 and the sensorarrangement 4. The sensor arrangement 4 is disposed on or coupled to thecylinder back-bias magnet 5 such that the center point between thesensor elements 20 (e.g., the center of circle 12) is aligned on thecenter axis 9 of the magnet 5. As described above, the sensorarrangement 4 (i.e., the sensor) includes magnetic sensor elements 20and an IC for signal conditioning.

FIG. 8 illustrates a plan view of another sensor module, including asensor arrangement arranged over a back-bias magnet, according to one ormore embodiments. In this case, three magnetic field sensor elements20L, 20R, and 20U are provided. The sensor elements 20 are arranged on acircumference of the zero-field circle 12 with equidistant spacing fromeach other. Thus, the sensor elements 20 are spatially distributedequally about a center of the zero-field circle 12 such that all sensorelements 20 are exposed to substantially the same (due to typicalassembly tolerances of 3%), or exactly the same magnetic field workingpoint.

As described above, each point on the zero-field circle 12 experiences azero magnetic flux density. In a preferred case, all sensor elements 20are exposed to a zero in-plane magnetic field Bxy but may be exposed toa residual offset field if not place exactly on the zero-field circle12. As a result of their arrangement on the zero-field circle 12 that isgenerated due to the bore 10 or substantially proximate to thezero-field circle 12, all sensor elements are always operated in linearmode (i.e., non-saturated mode) regardless of the overall magnetizationstrength of the magnet 5.

For each sensing element 20, the corresponding sensing axis is alignedin a radial or anti-radial direction of the in-plane field and areinsensitive to other magnetic in-plan field components. The threesensing signals from the three sensing elements 20L, 20R, and 20U can becombined in arbitrary way by the sensor circuit 21 to generate an outputsignal.

From the three sensor elements 20, three differential signals areobtained by the sensor circuit 21, including Δ1=BradialUP−BradialLEFT,Δ2=BradialLEFT−BradialRIGHT, Δ3=BradialRIGHT−BradialUP, where BradialUPis the sensor signal generated by sensor element 20U, BradialLEFT is thesensor signal generated by sensor element 20L, and BradialRIGHT is thesensor signal generated by sensor element 20R. The three differentialsignals may then be added by the sensor circuit 21 to generate thesensor output signal SE_(OUT).

The sensor signals of each sensor element 20 is provided to the sensorcircuit 21 that calculates the three differential signals Δ1, Δ2 and Δ3and/or an output signal using a differential calculation, andout-of-plane magnetic field components do not affect the output signal(i.e., the sensor output).

FIG. 9 illustrates a plan view of another sensor module, including asensor arrangement arranged over a back-bias magnet, according to one ormore embodiments. In this case, three magnetic field sensor elements20L, 20R, and 20C are provided. The sensor elements 20 are linearlyarranged with respect to each other with two of the sensor elements 20Land 20R arranged on a circumference of the zero-field circle 12 withequidistant spacing from the center sensor element 20C. The centersensor element 20C is arranged on an extension of the center axis 9 ofthe bore 10, which also coincides with the center of the innercircumference 7, the center of the zero-field circle 12, and the centerof the outer circumference 8. In other words, each sensor element 20 isarranged in a location that has a zero-magnetic field offset where themagnetic flux density of the magnetic in-plane field produced by themagnet 5 is zero or substantially zero.

In this example, the sensor elements 20 are linearly arranged along thex-axis and each have a sensing axis aligned with the x-direction formeasuring the magnetic field component Bx. Alternatively, the sensingaxis may be aligned with the negative x-direction for measuring themagnetic field component −Bx. Alternatively, the sensor elements 20 maybe linearly arranged along the y-axis and each have a sensing axisaligned with either the positive y-direction or the negativey-direction. Alternatively, the sensor elements 20 may be linearlyarranged along some other (i.e., arbitrary) in-plane axis with theirsensing axis pointing in the same direction. In each case, the centersensor element 20C is arranged at an extension of the center axis 9 (atthe center of the zero-field circle 12) and the other sensor elementsare arranged equidistant therefrom on the zero-field circle 12.

Sensor signals generated from the two outer sensor elements 20 (i.e.,sensor elements 20L and 20R) are received by the sensor circuit 21 andmay be used thereby to generate a first differential measurement signal(e.g., Δ1=Left−Right or in a magnetic field representation,Δ1=BxLeft−BxRight). Additionally, the sensor signal generated by thecenter sensor element 20C are received by the sensor circuit 21 and maybe used thereby as a second measurement signal SEM for generating asecond differential measurement signal SE_(OUT) (e.g.,SE_(OUT)=Center−(Left+Right)/2. The signals Δ1 and SE_(OUT) are 90°phase shifted to each other and can be used to determine speed androtation direction of the rotating target. For example, signal Δ1 may berepresentative of the rotational speed and the sign (positive ornegative) of the phase shift between signals Δ1 and SE_(OUT) may beindicative of the rotation direction.

As described above, each point on the zero-field circle 12 experiences azero in-plane magnetic flux density. In a preferred case, all sensorelements 20 are exposed to a zero in-plane magnetic field Bxy. As aresult of their arrangement on the zero-field circle 12 that isgenerated due to the bore 10 or substantially proximate to thezero-field circle 12, all sensor elements are always operated in linearmode (i.e., non-saturated mode) regardless of the overall magnetizationstrength of the magnet 5. As before, each sensor's transfer function hasa high linear range (minimum of +/−25 mT) and is in a wide rangeindependent from bias fields. That is, each sensor element 20 issensitive to a first magnetic in-plane field component (e.g., a Bxcomponent or By component) and, at the same time, it is independent from(or insensitive to) a second, different magnetic in-plan field component(e.g., a By component or a Bx component).

In view of the above embodiments, at least two sensor elements 20 areprovided in the sensor arrangement 4, where two or more of the sensorelements 20 are arranged on the zero-field circle 12. In some cases, oneadditional sensor element 20C is arranged at an extension of the centeraxis 9 of the magnet 5 at the center point of the zero-field circle 12.The center sensor element 20C may also be used in combination with thesensor arrangements illustrated in FIGS. 5 and 8 . Alternatively, allsensor elements 20 of the sensor may arranged on the zero-field circle12.

FIGS. 10A-10D illustrate sensor signals of a sensor elements of a sensorshown in FIG. 5 verse a rotation angle of a target wheel according toone or more embodiments. Additionally, FIG. 10E illustrates outputsensor signals of a sensor circuit of a sensor shown in FIG. 5 verse arotation angle of a target wheel according to one or more embodiments.

In other words, the sensor signals shown in FIG. 10A are representativeof sensor signal SE_(A) according to different airgaps between thesensor arrangement 4 and the toothed wheel 1, the sensor signals shownin FIG. 10B are representative of sensor signal SE_(B) according todifferent airgaps between the sensor arrangement 4 and the toothed wheel1, the sensor signals shown in FIG. 10C are representative of sensorsignal SE_(C) according to different airgaps between the sensorarrangement 4 and the toothed wheel 1, the sensor signals shown in FIG.10D are representative of sensor signal SE_(D) according to differentairgaps between the sensor arrangement 4 and the toothed wheel 1, andthe output sensor signals shown in FIG. 10E are representative of thesensor output SE_(OUT) according to different airgaps between the sensorarrangement 4 and the toothed wheel 1 calculated according to Equation(5). The shape of the target wheel (tooth 2 and notches 3) isrepresented by a rectangular shaped function in each graph.

Additionally, FIG. 10F illustrates output sensor signals for differentTIM angles of the sensor module shown in FIG. 5 verse a rotation angleof a target wheel according to one or more embodiments. Different twistangles (0°, 22.5°, and 45°) of the sensor module 6 around its z-axis aresuperimposed for each air gap.

As can be seen, the twist of the sensor module 6 about the z-axis haslittle effect on the output signals and there is essentially no phasevariation induced by the TIM angle. In particular, the rotation of thetarget wheel modulates the magnetic field, and a clear signal change(modulation) as a function of the wheel rotation angle is generated bythe sensor circuit 21 independent of the TIM angle of the sensor module6. This phenomenon is observed from the nearly overlapping curves forthe different twist angles for each air gap shown in FIG. 10F.

In view of FIGS. 10A-10F, the output signal may be independent from themounting angle (i.e., independent of a twisting angle around itsz-axis). The sensor arrangement 4 may be robust against stray-fields dueto differential signal calculation that cancels out homogeneousstray-fields in both in-plane directions (i.e., the x and y-planes), andout-of-plane magnetic field components do not affect the output signal.The output signal of the sensor circuit 21 complies with outputswitching on the tooth edge. Thus, there is no need to reconfigure anexternal control unit (e.g., an ECU) during installation. Furthermore, asimple axially polarized cylinder back-bias magnet with a center bore orcavity is sufficient. Accordingly, the described embodiments offerstray-field robust, twist-insensitive sensing of the wheel, and it comeswith a low cost magnetic back-bias solution (e.g., a sintered ferritecylinder magnet). Alternatively, other types of magnets (e.g., a rareearth magnet) may also be suitable as a back-bias magnet.

The low magnetic field offset that can be seen by the sensor in theregion of the notch 3 of the tooth wheel 1 can also be useful for thelifetime stability and true power on feature. TPO means the sensorcircuit 21 is able to detect directly at startup the correct position ofthe wheel 1 (e.g., it detects whether there is a tooth or notch in frontof it). This is realized by a threshold value. If the sensed fieldexceeds the threshold, then there is a tooth. If the sensed field issmaller than the threshold, then there is a notch. In case of magnetdegradation over its lifetime (i.e., magnet loses its strength), themagnetic offset will still be at or close to zero. Thus, there is littleto no impact on the performance of the sensor or its capability todetect a tooth or notch at startup despite a degradation of the magnet.

FIG. 11 illustrates a flow diagram of a method 1100 of measuring arotational speed of a rotating member by a magnetic sensor according toone or more embodiments. As noted above, the magnetic sensor including aplurality of sensor elements arranged in a sensor plane of the magneticsensor and are exposed to substantially the same working point of aradially symmetric bias magnetic field produced by an axially polarizedback-bias magnet that has a bore.

The method includes generating measurement values by a plurality ofsensor elements in response to sensing the radially symmetric biasmagnetic field (operation S5). The variations in the measurement valuesof the plurality of sensor elements are caused by a rotation of therotating member.

The method 1100 further includes generating a measurement signal usingat least one differential calculation with the measurement values asinputs to the differential calculation (operation S10). The differentialcalculation, performed by a processor, is configured to, based on themeasurement values, cancel out stray-fields along at least onesensitivity direction. For example, a first pair of sensor elements aresensitive to a first in-plane magnetic field component of the radiallysymmetric bias magnetic field in the direction of the firstsensitivity-axis, and a second pair of sensor elements are sensitive toa second in-plane magnetic field component of the radially symmetricbias magnetic field in the direction of the second sensitivity-axis.Accordingly, the measurement signal oscillates between maximum andminimum values based on a rotational speed of the rotating member.

Lastly, the method 1100 includes outputting the measurement signal to anexternal device (operation S15), such as an ECU, for further processing.The measurement signal may be output by transmission along a wiredconnection or a wireless connection.

Assembly and magnetization tilt errors may result in the sensor elementsbeing shifted away from the zero-magnetic field offset where themagnetic flux density of the magnetic in-plane field produced by themagnet 5 is zero or substantially zero. For example, an assembly errormay result in the sensor elements 20 being places too far from thecenter axis 9 or the zero-field circle 12. Production errors duringmanufacturing of the magnet 5 may result is magnetization tilt errors,which may cause the zero-magnetic field offset to be misaligned with thecenter axis and may cause the zero-field circle 12 to shift and/or beasymmetric with respect to the center axis 9. Of course, a combinationof assembly and production errors is also possible, leading to thehigher likelihood that the sensor elements 20 will be placed in aback-bias magnetic field that is greater than zero, greater than 15 mT,and/or that is outside of their respective linear range.

FIG. 12 illustrates a sensor module 6 including a shim polepieceaccording to one or more embodiments. The sensor module 6 includes theback-bias magnet 5 that is an axially polarized cylinder and includes abore 10, an inner diameter 7, and an outer diameter 8. The back-biasmagnet 5 includes a first end 25 and a second end 26 arranged oppositeto the first end 25 in the axial direction. The bore 10 extends from thefirst end 25 at least partially towards the second end 26 if not fullythrough the back-bias magnet 5 to the second end 26. The bore 10 definesthe inner diameter 7 (i.e., inner perimeter or inner boundary) of theback-bias magnet 5.

The sensor module 6 also includes a sensor arrangement 4 represented bya sensor plane (XY plane) in the figure.

The sensor module 6 further includes a shim polepiece 30 mechanicallycoupled to the first end 25 of the back-bias magnet 5 (i.e., the endmost proximate to the sensor arrangement 4) and interposed between theback-bias magnet 5 and the sensor arrangement 4. For this reason, thefirst end 25 may be referred to as the sensor side of the back-biasmagnet 5. The polepiece 30 is made of soft magnetic material, such asnickel iron NiFe, and has a disc shape in the present example. However,just as other shapes for the back-bias magnet 5, the same applies to thepolepiece 30. More importantly, the polepiece 30 includes a bore 31 thatextends through the thickness of the polepiece 30. The bore 31 definesan inner diameter 32 (i.e., inner perimeter or inner boundary) of thepolepiece 30. The center axis of the polepiece 30 is aligned with andcongruent (i.e., matched) with the center axis 9 of the back-bias magnet5. The inner diameter 32 of the polepiece 30 is aligned with andcongruent (i.e., matched) with the inner diameter 7 (i.e., innerperimeter or inner boundary) of the back-bias magnet 5. The outerdiameter 33 (i.e., outer perimeter or outer boundary) of the polepiece30 is aligned with and congruent (i.e., matched) with the outer diameter8 (i.e., outer perimeter or outer boundary) of the back-bias magnet 5.In other words, the shape profile of the polepiece 30 in a plan view ismatched with the shape profile of the back-bias magnet 5.

FIG. 13A illustrates the in-plane magnetic field components Bx and By ofan ideal axially polarized back-bias magnet 5 comprising a bore 10 andwithout the polepiece 30. The ideal sensor element locations of senorelements 20L, 20C, and 20R are shown at an intersection of in-planemagnetic field components Bx and By. This intersection at 0 mT for bothmagnetic field components Bx and By represents a location in the sensorplane at which there is a zero-magnetic field offset (i.e., where bothBx and By are equal to 0 mT). The locations of the zero-magnetic fieldoffsets are indicated by ZFL, ZFC, and ZFR, respectively, and arealigned with 20L, 20C, and 20R. The locations of the zero-magnetic fieldoffsets ZFL and ZFR correspond to the location of the zero-field circle12 described above (see e.g., FIGS. 2, 5, 8, and 9 ).

In particular, sensor element 20C is arranged at or aligned with thecenter axis 9 and sensor elements 20L and 20R are arranged on thezero-field circle 12 in accordance with FIG. 9 . Markers m5 and m6indicate an example assembly tolerance range for sensor element 20L atwhich the magnetic field offset is substantially zero. Markers m3 and m4indicate an example assembly tolerance range for sensor element 20C atwhich the magnetic field offset is substantially zero. Markers m7 and m8indicate an example assembly tolerance range for sensor element 20R atwhich the magnetic field offset is substantially zero. An assemblytolerance range is within the linear range of the sensor element.

FIG. 13B illustrates the in-plane magnetic field components Bx and By ofa tilted axially polarized back-bias magnet 5 comprising a bore 10 andwithout the polepiece 30. Here, due to a production error, themagnetization of the back-bias magnet 5 is tilted 10° with respect tothe center axis 9 instead of being aligned therewith. The locations ofthe zero-magnetic field offsets ZFL, ZFC, and ZFR are shifted from theirrespective sensor elements 20L, 20C, and 20R such that 20L, 20C, and 20Rare no longer located in a zero-magnetic field offset of a zero orsubstantially zero (<15 mT) in-plane magnetic field for both Bx and By.Accordingly, without the polepiece 30, an alignment issue between sensorlocations and zero-magnetic field offset locations exists due to thetilt in the axial magnetization. In addition, the tilted magnetizationcauses the zero-field circle 12 to become elliptical, oblong, skewed,shifted, or otherwise have a misshapen shape. In this case, thezero-field circle 12 may be more generally referred to as a zero-fieldclosed loop that encircles the center axis 9 and may have an elliptical,oblong, skewed, shifted, or otherwise misshapen shape.

FIG. 14A illustrates the in-plane magnetic field components Bx and By ofan ideal axially polarized back-bias magnet 5 comprising a bore 10 andwith the polepiece 30. The ideal sensor element locations of senorelements 20L, 20C, and 20R are shown at an intersection of in-planemagnetic field components Bx and By. It is important to note that slopeof the in-plane magnetic field components Bx and By are flatter due tothe polepiece 30 while still achieving intersecting in-plane magneticfield components Bx and By at 0 mT. This in effect increases theassembly tolerance ranges m5 to m6, m3 to m4, and m7 to m8, which allowsfor higher assembly tolerances with respect to the placement of sensorelements 20L, 20C, and 20R to be placed in a zero-magnetic field offsetof a zero or a substantially zero (<15 mT) in-plane magnetic field forboth Bx and By. Thus, the assembly tolerance is relaxed and reducesassembly errors.

Due to the polepiece 30 with bore 31 that is congruent with bore 10, thepolepiece 30 homogenizes the magnetic field, producing a lower gradientor slope. Furthermore, unlike solid polepieces that do not have a boreand do not have a shape profile that matches that of the magnet 5, threelocations of zero-magnetic field offsets ZFL, ZFC, and ZFR aremaintained with the polepiece 30.

FIG. 14B illustrates the in-plane magnetic field components Bx and By ofa tilted axially polarized back-bias magnet 5 comprising a bore 10 andwith the polepiece 30. Here, due to a production error, themagnetization of the back-bias magnet 5 is tilted 10° with respect tothe center axis 9 instead of being aligned therewith. However, unlikethe situation illustrated in FIG. 13B, the locations of zero-magneticfield offsets ZFL, ZFC, and ZFR are shifted only slightly from theirrespective sensor element locations. In addition, the slope of thein-plane magnetic field components Bx and By are flatter due to thepolepiece 30, which increases the assembly tolerance range. Furthermore,unlike solid polepieces that do not have a bore and do not have a shapeprofile that matches that of the magnet 5, three locations ofzero-magnetic field offsets ZFL, ZFC, and ZFR are still produced withthe polepiece 30. Zero-magnetic field offsets ZFL and ZFR do not occurat all when using a solid polepiece.

The polepiece 30 produces a lower magnetic field gradient or slope atthe sensor plane. The polepiece 30 homogenizes the in-plane magneticfield components Bx and By produced at the sensor plane. The polepiece30 ensures that the zero-magnetic field offsets ZFL and ZFR are locatedbetween the inner diameter 7 and the outer diameter 8 of the magnetic 5from a plan view, which is not possible regardless of where the sensorplane is located when using a solid polepiece. In other words, itensures the locations of the zero-magnetic field offsets ZFL and ZFR(e.g., of the skewed and/or misshaped zero-field circle 12) aremaintained over the body of the magnet 5 in a plan view, as shown inFIGS. 2, 5, 8, and 9 . In essence, the polepiece 30 with bore 31 andcongruent shape profile realigns the tilted magnetization towards thecenter axis 9, thereby shifting the center zero-magnetic field offsetZFC towards the center axis 9. Or said differently, the polepiece 30rectifies the alignment of the in-plane magnetic field components Bx andBy towards the axial direction of the magnet 5. The polepiece 30 alsoensures that the location of the zero-magnetic field offset ZFC ismaintained within an assembly tolerance range from the center axis ofthe magnet 5 such that sensor element 20C experiences zero orsubstantially zero magnetic field from in-plane components. Thepolepiece 30 also ensures that the locations of the zero-magnetic fieldoffsets ZFL and ZFR are maintained within an assembly tolerance rangefor sensor elements 20L and 20R such that they experiences zero orsubstantially zero magnetic field from in-plane components.

FIG. 15A illustrates the in-plane magnetic field components Bx and By ofan ideal axially polarized back-bias magnet comprising a bore and with aconventional solid polepiece. FIG. 15B illustrates the in-plane magneticfield components Bx and By of a tilted axially polarized back-biasmagnet comprising a bore and with a conventional solid polepiece. Inboth instances, zero-magnetic field offsets ZFL and ZFR are not presentand do not occur in this set up. Thus, sensor elements 20L and 20R maybe placed in their saturation ranges (i.e., outside of their linearranges), making the setup unreliable. The conventional solid polepieceeffectively nullifies the benefits of the back-bias magnet 5, includingthe generation of zero-magnetic field offsets ZFL and ZFR as describedherein.

In view of the above, assembly tolerances can be relaxed by implementingthe sensor modules described above due to their twist-insensitivity andstray-field robustness.

While the above embodiments are described in the context of detecting awheel or camshaft speed, the sensor may be used to detect the rotationspeed of any rotating member or object that creates sinusoidalvariations in a magnetic field as it rotates and that may be sensed by asensor, including a crankshaft and transmission speed sensing. Forexample, a combination of a ferrous wheel and a back-bias magnet may beused to generate a time varying magnetic field.

Further, while various embodiments have been described, it will beapparent to those of ordinary skill in the art that many moreembodiments and implementations are possible within the scope of theinvention. Accordingly, the invention is not to be restricted except inlight of the attached claims and their equivalents. With regard to thevarious functions performed by the components or structures describedabove (assemblies, devices, circuits, systems, etc.), the terms(including a reference to a “means”) used to describe such componentsare intended to correspond, unless otherwise indicated, to any componentor structure that performs the specified function of the describedcomponent (i.e., that is functionally equivalent), even if notstructurally equivalent to the disclosed structure that performs thefunction in the exemplary implementations of the invention illustratedherein.

Furthermore, the following claims are hereby incorporated into thedetailed description, where each claim may stand on its own as aseparate example embodiment. While each claim may stand on its own as aseparate example embodiment, it is to be noted that—although a dependentclaim may refer in the claims to a specific combination with one or moreother claims—other example embodiments may also include a combination ofthe dependent claim with the subject matter of each other dependent orindependent claim. Such combinations are proposed herein unless it isstated that a specific combination is not intended. Furthermore, it isintended to include also features of a claim to any other independentclaim even if this claim is not directly made dependent to theindependent claim.

It is further to be noted that methods disclosed in the specification orin the claims may be implemented by a device having means for performingeach of the respective acts of these methods.

Further, it is to be understood that the disclosure of multiple acts orfunctions disclosed in the specification or claims may not be construedas to be within the specific order. Therefore, the disclosure ofmultiple acts or functions will not limit these to a particular orderunless such acts or functions are not interchangeable for technicalreasons. Furthermore, in some embodiments a single act may include ormay be broken into multiple sub acts. Such sub acts may be included andpart of the disclosure of this single act unless explicitly excluded.

What is claimed is:
 1. A magnetic sensor module, comprising: an axiallypolarized back-bias magnet comprising a magnet body that extends in aradial direction from an inner sidewall to an outer sidewall and a firstbore that defines the inner sidewall, wherein the first bore is centeredon a magnet center axis of the axially polarized back-bias magnet andextends along the magnet center axis in an axial direction of theaxially polarized back-bias magnet, wherein the axially polarizedback-bias magnet comprises a first end and a second end arrangedopposite to the first end in the axial direction, wherein the axiallypolarized back-bias magnet generates a radial bias magnetic fieldcomponent about a magnetization axis of the axially polarized back-biasmagnet in a sensor plane, wherein the sensor plane is orthogonal to anextension of the magnet center axis, wherein the radial bias magneticfield component has a magnetic flux density of substantially zero alonga perimeter of a zero-field closed loop located in the sensor plane,wherein the perimeter of the zero-field closed loop is offset from theextension of the magnet center axis in the radial direction; and a shimpolepiece mechanically coupled to the first end of the axially polarizedback-bias magnet, wherein the shim polepiece comprises a second borecentered on a polepiece center axis that is aligned with the magnetcenter axis, wherein the second bore extends through the shim polepiecealong the polepiece center axis.
 2. The magnetic sensor module of claim1, wherein the axially polarized back-bias magnet, in combination withthe shim polepiece, generates the zero-field closed loop in the sensorplane.
 3. The magnetic sensor module of claim 2, wherein the shimpolepiece is configured to rectify the radial bias magnetic fieldcomponent such that the perimeter of the zero-field closed loopvertically overlaps with the magnet body in a plan view.
 4. The magneticsensor module of claim 3, wherein the magnetization axis is tilted awayfrom the magnet center axis and the shim polepiece is configured toshift the magnetization axis towards the magnet center axis of theaxially polarized back-bias magnet.
 5. The magnetic sensor module ofclaim 1, wherein the radial bias magnetic field component has a magneticflux density of substantially zero along the extension of themagnetization axis, and wherein the magnetization axis is tilted awayfrom the magnet center axis and the shim polepiece is configured toshift the magnetization axis towards the magnet center axis of theaxially polarized back-bias magnet.
 6. The magnetic sensor module ofclaim 5, wherein the shim polepiece is configured to rectify the radialbias magnetic field component such that the perimeter of the zero-fieldclosed loop vertically overlaps with the magnet body in a plan view. 7.The magnetic sensor module of claim 1, wherein the radial bias magneticfield component has a magnetic flux density of substantially zero alongthe extension of the magnetization axis, and the shim polepiece isconfigured to rectify the radial bias magnetic field component such thatmagnetization axis is maintained at the magnet center axis.
 8. Themagnetic sensor module of claim 1, wherein the shim polepiece isconfigured to homogenize a first in-plane magnetic field component and asecond in-plane magnetic field component of the radial bias magneticfield component.
 9. The magnetic sensor module of claim 1, wherein theshim polepiece is configured to reduce a gradient of the radial biasmagnetic field component at the magnetization axis and at the perimeterof the zero-field closed loop.
 10. The magnetic sensor module of claim1, wherein the radial bias magnetic field component comprises a firstin-plane magnetic field component and a second in-plane magnetic fieldcomponent, and the shim polepiece is configured to reduce a gradient ofthe first in-plane magnetic field component and the second in-planemagnetic field component at the magnetization axis and at the perimeterof the zero-field closed loop.
 11. The magnetic sensor module of claim1, wherein a shape profile of the shim polepiece is substantially thesame as a shape profile of the axially polarized back-bias magnet in aplan view.
 12. The magnetic sensor module of claim 1, wherein: theaxially polarized back-bias magnet comprises a magnet inner perimeterdefined by the first bore and a magnet outer perimeter, and the shimpolepiece comprises a polepiece inner perimeter congruent with themagnet inner perimeter and a polepiece outer perimeter congruent withthe magnet outer perimeter.
 13. The magnetic sensor module of claim 1,further comprising: a magnetic sensor including a plurality of sensorelements arranged in the sensor plane of the magnetic sensor atlocations where the radial bias magnetic field component has a magneticflux density of substantially zero, wherein the plurality of sensorelements are configured to generate measurement values in response tosensing the radial bias magnetic field component, wherein at least twoof the plurality of sensor elements are arranged on the perimeter of thezero-field closed loop at substantially equidistant angles about themagnet center axis of the axially polarized back-bias magnet.
 14. Themagnetic sensor module of claim 13, wherein the radial bias magneticfield component has a magnetic flux density of substantially zero alongthe extension of the magnetization axis and one sensor element of theplurality of sensor elements is arranged on the extension of themagnetization axis.
 15. The magnetic sensor module of claim 14, whereinthree sensor elements of the plurality of sensor elements are linearlyarranged along an in-plane axis of the sensor plane, wherein thein-plane axis is orthogonal to the magnet center axis.
 16. The magneticsensor module of claim 13, wherein the magnetic flux density ofsubstantially zero means less than 15 milliteslas (mT).
 17. The magneticsensor module of claim 16, wherein the plurality of sensor elements arearranged in the sensor plane of the magnetic sensor at locations wherethe radial bias magnetic field component has a magnetic flux density ofsubstantially zero.
 18. The magnetic sensor module of claim 1, whereinthe perimeter of the zero-field closed loop is defined as an area in thesensor plane where the magnetic flux density of the radial bias magneticfield component is less than 15 milliteslas (mT).
 19. The magneticsensor module of claim 1, wherein the axially polarized back-biasmagnet, in combination with the shim polepiece, generates the radialbias magnetic field component that is zero at the magnetization axis andthe extension thereof, increases in the radial direction in the sensorplane from the magnetization axis to a first radial distance, decreasesin the radial direction in the sensor plane from the first radialdistance to a second radial distance that defines a portion of theperimeter of the zero-field closed loop, and increases in the radialdirection in the sensor plane from the second radial distance to a thirdradial distance that defines the outer sidewall.
 20. A magnetic sensormodule, comprising: an axially polarized back-bias magnet comprising amagnet body that extends in a radial direction from an inner sidewall toan outer sidewall and a first bore that defines the inner sidewall,wherein the first bore extends along a magnet center axis in an axialdirection of the axially polarized back-bias magnet, wherein the axiallypolarized back-bias magnet comprises a first end and a second endarranged opposite to the first end in the axial direction, wherein theaxially polarized back-bias magnet generates a radial bias magneticfield component about a magnetization axis of the axially polarizedback-bias magnet in a sensor plane, wherein the sensor plane isorthogonal to an extension of the magnet center axis, wherein the radialbias magnetic field component has a magnetic flux density ofsubstantially zero at a zero-field closed loop located in the sensorplane, wherein the zero-field closed loop encircles the extension of themagnet center axis; and a shim polepiece mechanically coupled to thefirst end of the axially polarized back-bias magnet, wherein the shimpolepiece comprises a second bore centered on a polepiece center axisthat is aligned with the magnet center axis, wherein the second boreextends through the shim polepiece along the polepiece center axis.